Equal-loudness contours, phon and sone
Our ears are not equally sensitive at all frequencies, and the sensitivity changes with level. Equal-loudness contours map this, and they explain why music sounds thin when it is played quietly.
What a contour shows
Pick a 1 kHz tone at some level, say 40 dB SPL. Play tones of other frequencies and adjust each until it sounds as loud as the 1 kHz tone. The levels you find form one curve: the 40 phon contour. The unit phon is defined as the sound pressure level of a 1 kHz tone that sounds equally loud, so phon and dB SPL agree at 1 kHz and only there.
The curves show:
- The ear is most sensitive between about 2 and 5 kHz, where the ear canal resonates.
- At low frequencies much higher levels are needed. At 40 phon, roughly 64 dB at 100 Hz and about 78 dB at 50 Hz sound as loud as 40 dB at 1 kHz.
- At high loudness the curves flatten: the bass catches up. At 80 phon, the differences are much smaller than at 40 phon.
Consequences
- Loudness changes the tonal balance. Quiet playback sounds weaker in the bass and the highest treble than loud playback of the same signal.
- Loudness controls on consumer equipment lift the bass and treble at low volume to compensate.
- Mixing level matters. A mix balanced at low level can sound bass-heavy when played loudly and vice versa, which is a reason to check at several levels.
Three families of curves
| Source | Year | Note |
|---|---|---|
| Fletcher and Munson | 1933 | The first widely used set, based on early experiments |
| Robinson and Dadson | 1956 | The basis of the ISO standard of 1987 |
| ISO 226 | 2003 | A revision from newer studies, with notable differences in the low and mid frequencies |
The earlier sets are often wrongly mixed. "Fletcher-Munson curves" is used as a generic name for all of them, but the 2003 standard is the current reference.
Loudness in sone
Sone is a linear scale of loudness: 1 sone is the loudness of a 40 phon tone, and each 10 phon increase doubles the sone value. A 10 dB increase at mid levels is roughly twice as loud, see the loudness tool.
Weighting filters
Measurement filters approximate the contours. A-weighting follows roughly the inverse of the 40 phon curve, C-weighting a high-level curve, see A, B, C and Z weighting.
Limits
Contours are averages for young listeners with normal hearing, measured with pure tones in a free field. Individuals differ, and complex sounds do not follow the curves exactly.
Draft: written by an AI assistant, awaiting technical review.
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